1. Field of the Invention
This invention relates to a metal cord for reinforcing a rubber article such as a vehicle tire, a conveyor belt or a high-pressure hose, and more particularly a metal cord which is effective in improving the durability of such rubber articles.
2. State of the Prior Art
A metal cord for reinforcing a rubber article is usually made of high carbon steel wires containing 0.65-0.85% of carbon (JIS G 3502 piano wire rods) having their surface plated with such metals as brass, copper or zinc in order to impart adhesion with rubber. Such wires are drawn to a diameter of 0.1 to 0.4 mm and twisted together.
The quality characteristics required for a metal cord for reinforcing a vehicle radial tire includes adhesion with rubber, bonding durability, resistance to corrosion and various mechanical properties (such as breaking load, rigidity, fatigue properties and flexibility). Among them, the resistance to corrosion is greatly influenced by the penetration of rubber into the cord.
Among the metal cords having a single twisting structure, closed cords (C.C.) having 1.times.5, 1.times.4 and 1.times.3 structures are widely used. But as shown in FIG. 5, which show such cords in section, they have a hollow space in the center. If such a cord is used for reinforcing a rubber article, rubber cannot penetrate into the hollow space. Since rubber penetration into the cord is poor, if the tire should hit a stone or a nail and sustain a cut so deep as to reach the metal cord, water will penetrate into the cord, developing corrosion from inside the cord. This will lower the breaking load and the resistance to fatigue. Also, the bonding strength between the cord and the rubber will decrease, causing peeling and thus separation of the tire. This will cause the trouble with quality.
In order to improve rubber penetration, an open cord (O.C.) as shown in FIG. 6 was proposed in Unexamined Japanese Patent Publication 55-90692 in which metal filaments 1 are twisted together with spaces formed therebetween. Further, in Examined Japanese Patent Publication 58-31438, a cord as shown in FIG. 7 was proposed having a cross-section which is not uniform in the longitudinal direction (hereinafter referred to as 2+2 cord).
Various other cords are known, which will be described later.
The aforementioned open cord and the 2+2 twisting structure were proposed in order to improve rubber penetration into the cord. But since the open cord is liable to become stretched under low tensile load, when tension is applied to such cords during the calendering step in the production of a tire, they will become stretched and their metal filaments will be drawn close to one another. This may reduce the size of the spaces between the filaments, thus causing insufficient rubber penetration. Also, these cords have a problem in that when the cord is compressed under pressure during the vulcanizing step after forming a tire, the spaces between the filaments tend to be reduced, making the penetration of rubber difficult. Further, in producing such open cords, it is extremely difficult to obtain a cord having a uniform twisting structure in the longitudinal direction, since the twisting is loose. Thus, the spaces between the metal filaments may vary. Also, the metal filaments may be distributed not uniformly, but partially to one side with respect to the diametric direction of the cord as viewed in section. This will cause individual cords to be stretched unevenly due to tension loads when pulling the cords for proper arrangement during the calendering step. Thus, the distances between the cords tend to be uneven.
On the other hand, the 2+2 cord permits good rubber penetration. But this cord has a problem in that its cross-sectional shape is not circular but rugged at parts in the longitudinal direction. The distances between cords tend to fluctuate and become uneven when they pass through guide rollers during the calendering step. Further, the resistance to fatigue is bad.